The field of art to which this invention is directed is hydrogel-forming polymer compositions made from crosslinked polyacrylic acid.
Water-insoluble hydrogel-forming polymers which are capable of absorbing large quantities of water and aqueous fluid are well known compositions. Such polymers, which are known as superabsorbent polymers, are lightly crosslinked acid functional polymers which swell in water or aqueous fluids but do not dissolve in the fluids. Superabsorbent polymers have been found to be particularly useful in diapers, feminine hygiene articles and surgical dressings. Descriptions of superabsorbent polymers and their uses are found in U.S. Pat. Nos. 3,669,103 and 3,670,731.
U.S. Pat. No. 4,654,039 (Reissue 32,649) is directed to hydrogel forming polymer compositions which are described as substantially water-insoluble, slightly cross-linked, partially neutralized polymers, derived from unsaturated polymerizable, acid group-containing monomers and cross-linking agents. Such polymers are made by polymerizing the acid monomer and the crosslinking monomer in water using a redox initiator system, followed by partially neutralizing the acid groups with sodium hydroxide, then drying the polymer and pulverizing it to a powder.
British Patent No. 2,119,384 discloses superabsorbent polymers made by polymerizing in water acrylic acid in admixture with sodium acrylate and a crosslinking monomer using a persulfate initiator, followed by drying and then heating with a crosslinking agent having at least two functional groups capable of reacting with carboxyl groups.
In U.S. Pat. No. 4,497,930, superabsorbent polymers are made by polymerizing acrylic acid in an inverse emulsion process followed by crosslinking the polymer with a diepoxide compound.
According to U.S. Pat. No. 4,295,987, superabsorbent polymers are made by polymerizing acrylic acid and a multi-functional acrylate monomer in water using a persulfate initiator followed by neutralizing the acid groups with caustic and then blending in a divalent cation salt, e.g., zinc acetate, for additional crosslinking.
Numerous other patents disclose superabsorbent polymers and their uses, such as U.S. Pat. Nos. 4,076,663, 4,552,938, 4,507,438, 4,535,098, 4,820,773 and European Patent Application 189,163.
Many improvements have been made in the performance and properties of superabsorbent polymers over the years, such as in gel strength and absorbing capacity. However, such superabsorbent polymers do not have a balance of properties. Typically polymers with high elasticity modulus exhibit reduced absorbing capacity, which results, for example, in reduced diaper dryness. Polymers with increased absorbing capacity exhibit low absorbency under pressure and a reduced elasticity modulus which also reduces diaper dryness. European Patent Application 339,461 describes absorbent products made with superabsorbent polymers having the ability to swell against pressure.
There is a need for a superabsorbent polymer having a balance of properties which, when the polymer is used in a diaper, results in improved diaper dryness and leakage.